The use of distributed Raman amplification in transmission fibers permits a significant improvement in the characteristics of optical transmission systems. For example, for a given optical signal-to-noise ratio at the end of a link, the use of this technique enables the length of the individual link sections to be increased or more link sections can be bridged.
When the technique is used in WDM systems, the Raman gain should have a flat gain spectrum, so that all the channels benefit to the same extent. On the other hand, the system improvement which can be obtained is limited by the channel with the lowest gain. The higher gain of the other channels corresponds to an inefficient utilization of the pump power deployed and, if the differences are very large, can degrade their signal quality by doubled Rayleigh backscattering.
A flat gain spectrum across a wide range of wavelengths can be obtained by the use of several pumping signals at different wavelengths. However, the desired gain spectrum is achieved only for a quite particular distribution of the power at the individual pumping wavelengths. These must be adjusted for the required gain, the position of the pumping wavelengths relative to the signal wavelengths, the insertion loss between the pumping source and the transmission fiber, and the characteristics of the transmission fiber.
The characteristics of the transmission fiber which are relevant for the Raman amplification can have such a wide distribution from one sample to another, even for fibers of one type (SSMF, LEAF, TrueWave, . . . ), that there are detectable differences in the resulting gain spectrum. In addition, when the system is installed nothing is generally known about the insertion loss between the pumping source and the actual input to the transmission fiber. It is therefore only possible to set up a desired gain spectrum when the system is being commissioned if the actual gain spectrum which applies over each link section can be measured, and the pumping powers appropriately adjusted if there are deviations.
Until now there have been essentially four known methods for setting the pumping powers of the Raman sources when a system is being commissioned. The first method can be used if the system is commissioned with its maximum number of channels. In this case, the link sections are started up one after another, starting with the one immediately after the transmitter. As all the channels are already present during commissioning, they can be used as a test signal spectrum for a gain measurement. The signal spectrum at the output from the link section concerned is first measured with the Raman pumping source switched off, then the spectrum with the source switched on. The ratio of the two spectra, or the difference in the level in dB, as applicable, immediately gives the on/off gain spectrum of the Raman amplifier. An exemplary embodiment of this type is described in EP 1 130 825 A2. Here, the gain measurement is carried out as a function of a specific configuration of active channels, which is undesirable when the system is being commissioned.
Unfortunately this method can seldom be used in practice, because most systems are commissioned using only a very small number of channels, and only later are they upgraded. It would indeed be possible in principle to measure and adjust the gain spectrum using 2a 
only the signal channels already initially available, since the gain in the case of the channels which are still missing plays no part. It would then be necessary to remeasure and tune the gain spectrum before or during the commissioning of additional channels. The switching off of the Raman pumping source which this requires would disrupt the transmission of the channels which are already present. For this reason, even during the initial commissioning of the system the gain spectrum should be measured and tuned for all the channels which will be present at maximum capacity.
In the case of the second known method for adjusting the pump powers, those channels which are not present at initial commissioning are replaced by a laser source with tunable wavelength. The determination of the signal spectrum at the output from the link section with the Raman pumping source switched off or on, as appropriate, thus requires many individual measurements, between which the tunable laser source must be switched over to the next channel. Apart from requiring a significantly longer measurement time, the method also requires a facility for communicating with the source, so that the latter can be informed when it should set which wavelength. In addition, the preparation of the source and the coupling of its output signal into the transmission system can present problems.
In order to be able to forgo a tunable laser source, a third method for the measurement of the gain spectrum has been proposed, which manages completely without a test signal at the input to the link section. This method utilizes the effect that the stimulated emission which is responsible for the gain is invariably accompanied by the generation of spontaneous emissions. For this reason, the spectrum of the ASE generated by the Raman amplifier is measured, and an attempt made to calculate the gain spectrum from this. Since the relationship between the gain spectrum and the spectrum of the ASE is very complex for distributed Raman amplifiers, the calculation is very resource intensive and error-prone.
The fourth method proposed for adjusting the pumping powers of the Raman pumping sources does completely without a measurement of the gain spectrum, and adjusts the pumping powers solely by reference to the specified fiber type for the transmission fiber. Since the method therefore has no knowledge of either the exact fiber characteristics or the insertion loss between the pumping source and the transmission fiber, the resulting gain spectrum can deviate significantly from that actually desired.
Examples of the state of the art, in which some of the methods itemized above are used, are set out below.
WO 00/73826 A2 presents an optical transmission system which has various amplifier units connected in series. The amplifier functions are checked either in operation or by means of supplementary units by gain measurements at the wavelength of the signal, and the pumping powers are adjusted correspondingly. For test measurements of the amplifier gain, use is made of both broadband and narrowband light sources as the ASE source, depending on the number of channels to be investigated.
US 2002/0071173 A1 and US 2002/0044336 A1 also show optical amplifier units in a transmission link, which can be controlled through the pumping power, for which in each case a Raman amplifier is put in circuit upstream from the optical fiber amplifier. In US 2002/0071173 A1, the amplifier contains a wavelength-dependent tunable filter. In order to obtain a flat gain spectrum, the gain spectra of the amplifier unit are checked and the filter adjusted accordingly. In US 2002/0044336 A1, the amplifier module contains a unit which determines whether the input signal has been interrupted due to noise effects in the Raman amplifier. The pumping power of the Raman source is adjusted according to the signal detection, by which means the noise level can also be regulated.
US 2002/0054733A1 discloses a typical transmission section with a Raman amplifier and an optical fiber amplifier as a booster, upstream from the Raman amplifier, for which the Raman gain spectrum is determined by a comparison of the spectrum of the signals transmitted over the active channels against a stored spectrum. Using a regulating unit, the power of the pumping source for the Raman amplifier is varied in such a way that minimal deviations occur between the spectra which are compared, from which the Raman gain can be derived.